This invention relates to a quartz glass, in particular to a quartz glass for use as a structural material for a heat treating apparatus to be employed in a semiconductor manufacturing process, and to a method manufacturing the quartz glass. This invention also relates to a heat treating apparatus using the quartz and to a heat treating method using the quartz.
A heat treating apparatus is employed as one of the apparatus to be employed in a semiconductor manufacturing process. In the heat treating apparatus of this kind, a quartz glass is employed as a structural material for a furnace tube, a wafer boat, etc., because the employment of quartz glass is advantageous in various aspects, e.g. it is available in high purity or it is excellent in heat resistance.
Even if a quartz glass of a high purity is employed as a structural material, it is difficult to prevent the surface of a silicon wafer from being contaminated with a metal such as Cu and Fe when the silicon wafer is heat-treated at a high temperature of 1,000.degree. C. or more. The percent defective of a semiconductor device which is manufactured by making use of a silicon wafer containing a relatively large quantity of metallic contaminants originated from a heat treating furnace is higher than that of a semiconductor device manufactured by making use of a silicon wafer which is free from the metallic contaminants.
The causes for the contamination of a silicon wafer with metals in its treatment inside a heat treating furnace may be attributed to the following facts.
First of all, metallic impurities which are contained in various external members (such as a heater, liner tube made of silicon carbide, etc.) disposed outside the quartz glass furnace tube and are large in diffusion coefficient in quartz glass are caused to evaporate from the surfaces of these external members, the evaporated metallic impurities being subsequently adsorbed on the external wall of the quartz glass furnace tube. The metallic impurities thus adsorbed are then diffused into the interior of the quartz glass furnace tube to reach the inner wall of the furnace tube, from which the metallic impurities are caused to be desorbed whereby contaminating a silicon wafer with the desorbed metallic impurities (a first cause).
Secondary, metallic impurities originally contained in the quartz glass furnace tube are caused to diffuse up to the surface of the furnace tube, from which the metallic impurities are caused to be desorbed whereby contaminating a silicon wafer with the desorbed metallic impurities (a second cause).
For the purpose of minimizing the metallic contamination to be brought about by these causes, a method of improving the purity of bulk of the constituent members of heat treating apparatus has been mainly adopted. For example, in the case of the quartz glass furnace tube, the following methods have been adopted to improve the purity of the quartz glass.
Namely, a method of enhancing the purity of natural quartz crystal as a raw material for quartz glass (a first method); a method of employing silicon tetrachloride or a synthetic amorphous silica to be obtained by means of sol-gel method (a second method); a method of enhancing the purity of the ingot to be obtained by melting a raw material (a third method); and a method consisting of a combination of two or more of the aforementioned first to third methods (a fourth method).
It is possible according to these methods to decrease the content of metallic impurities in a quartz glass down to 0.3 ppm or less, or down to 0.1 ppm or less depending on the kinds of metallic impurities. When a quartz glass of such a high purity is employed as a structural material for a furnace tube, the metallic contamination (transfer) of a silicon wafer can be minimized, and hence the percent defective of a semiconductor device can be correspondingly minimized.
At present, although it is possible to manufacture a quartz glass having a metallic impurity content of 0.3 ppm (0.1 ppm), it is still impossible to manufacture a quartz glass which is completely free from metallic impurities, i.e. the technique of enhancing the purity of quartz glass is now being nearly deadlocked.
Even if it has become possible to manufacture a quartz glass which is completely free from metallic impurities so as to solve the problem of contamination with metallic impurities originating from the aforementioned second cause, it is still impossible to solve the problem of contamination with metallic impurities originating from the aforementioned first cause.
Further, in view of a recent trend to further increase the integration of integrated circuit, a higher cleanness in the process of manufacturing a 1G bit DRAM, etc. than at present is now demanded. In order to meet such a demand, it is required not only to minimize the metallic impurities in a quartz glass so as to manufacture a high-purity quartz glass, but also to prevent a high-purity quartz glass from being contaminated by metallic impurities coming from outside the quartz glass.
As mentioned above, according to the conventional heat treating apparatus, even if the purity of quartz glass to be employed as a structural material of a furnace tube is enhanced, it has been impossible to prevent the furnace tube (quartz glass) from being contaminated by metallic impurities coming from various members disposed outside the quartz glass, thus resulting in the contamination of a silicon wafer.